ε-Caprolactam is a precursor for the preparation of Nylon-6. Nylon-6 was first made in 1899 by heating 6-aminohexanoic acid. Commercially feasible synthesis of Nylon-6 from ε-caprolactam was discovered by Paul Schlack at I. G. Farbenindustrie in 1938. Currently, approximately 95% of the world's ε-caprolactam is produced from cyclohexanone oxime via the Beckmann rearrangement. The starting material for cyclohexanone can be cyclohexane, phenol, or benzene. Through a series of reductions and/or oxidations, cyclohexanone is formed. The latter is then reacted with a hydroxylamine salt usually the sulfate, to form the oxime and ammonium sulfate. The oxime is rearranged in concentrated sulfuric acid, and the resulting lactam sulfate salt is neutralized with ammonia to form ε-caprolactam and additional ammonium sulfate. Subsequently, pure ε-caprolactam is obtained through numerous separation and purification steps. The current process is extremely capital intensive and generates large quantities of waste.
An economically attractive method of production of caprolactam uses 6-aminocapronitrile as a precursor. U.S. Pat. No. 2,301,964 (E. I. Du Pont de Nemours & Company) discloses a liquid-phase method of producing lactams from aminonitriles and water. Hydrolysis and concurrent lactam formation proceed rapidly when aminonitrile is reacted in a weak aqueous solution. Temperatures of from about 200° C. to about 375° C. are employed. The aminonitrile and water are maintained at this reaction temperature for not more than 1 hour. The reaction is preferably catalyzed with hydrogen sulfide.
U.S. Pat. No. 2,357,484 (E. I. Du Pont de Nemours & Company) discloses a vapor-phase catalytic process for preparing N-substituted amides comprising passing a vaporized mixture of water and an aliphatic aminonitrile, containing at least one aminonitrile moiety, over a dehydration-type catalyst at a temperature of typically from about 150° C. to about 500° C. for not more than 1 minute. When an open-chain aliphatic aminonitrile is used, in which the amino and nitrile groups are separated by at least two carbon atoms in contiguous relation, the product obtained is a lactam.
U.S. Pat. No. 6,353,101 (BASF) discloses the hydrolytic cyclization of aminonitriles, 6-aminocapronitrile in particular, to lactams, ε-caprolactam in particular, in the vapor phase using metal oxide catalysts. The use of a single bed or a single bed divided into several trays with addition of reactants or inert gases has been disclosed. Additionally, the use of one or more reaction chambers, such as a multitubular reactor, has been suggested.
U.S. Pat. No. 6,069,246 (Rhodia) discloses a method of production of caprolactam from 6-aminocapronitrile and water by vapor phase hydrolytic cyclization, followed by purification of the caprolactam by distillation. In order to prevent formation of oligomers during the distillation step in the method, the product of reaction of 6-aminocapronitrile and water is cooled down to a temperature below about 150° C. and, if necessary, kept in storage before it is distilled. Both cooling and storage increases capital and operating cost of the process.
It would, therefore, be desirable to have a process of producing caprolactam from 6-aminocapronitrile in which no cooling or storage is required before distillation. The present invention provides such a process.